The word "novella" means a long short story or a short novel. Just what is deemed a novella varies with the publisher, but a 50-page short story or a 100-page novel would likely be deemed novellas.

A novella is a short novel; a narrative work of prose fiction somewhat longer than a short story but shorter than a novel. A common length is about 50 to 100 pages. The extra length is generally used for more character development than is possible in a short story, but without the much greater character and plot development of a novel. Novellas often are characterized by satire or moral teaching. Although the novella is a common literary genre in several European languages, it is rarely used in English. Americans generally would be most familar with the novellas of Franz Kafka, particularly The Metamorphosis and In the Penal Colony, or George Orwell's Animal Farm. Stephen King is one of the few American authors who often writes novellas, such as those adapted as Secret Window, The Shawshank Redemption and Stand by Me, although they are usually published in collections. Like the English word "novel", the English word "novella" derives from the Italian word "novella" (plural: "novelle"), for a tale, a piece of news. As the etymology suggests, novellas originally were news of town and country life worth repeating for amusement and edification. As a literary genre, the novella's origin lay in the early Renaissance literary work of the Italians and the French. Principally, by Giovanni Boccaccio (1313–1375), author of The Decameron (1353)—one hundred novelle told by ten people, seven women and three men, fleeing the Black Death by escaping from Florence to the Fiesole hills, in 1348; and by the French Queen, Marguerite de Navarre (1492–1549), [aka Marguerite de Valois, et. alii.], author of Heptaméron (1559)—seventy-two original French tales (structured like The Decameron). Her psychological acuity and didactic purpose outweigh the unfinished collection's weak literary style. Not until the late eighteenth- and early nineteenth- centuries did writers fashion the novella into a literary genre structured by precepts and rules. Contemporaneously, the Germans were the most active writers of the Novelle (German: "Novelle"; plural: "Novellen"). For the German writer, a novella is a fictional narrative of indeterminate length—a few pages to hundreds—restricted to a single, suspenseful event, situation, or conflict leading to an unexpected turning point (Wendepunkt), provoking a logical, but surprising end; Novellen tend to contain a concrete symbol, which is the narration's steady point. http://en.wikipedia.org/wiki/Novella

Novellas have become quite common in genre fiction. They allow a publisher to package several, two to three, new or weak authors with a well-known author. Readers will buy the book for that author but may enjoy any or all of the other authors. The hope is that they will then buy the other authors' books as a result. These novellas are usually between 20,000 and 40,000 words. It's a good way for new writers to break into the market.